631
The rate-determining step of the Ramberg-Backlund rearrangement l 5 and related cyclopropane forming addition-eliminations16 may present such cases. In the former reaction an intramolecular s N 2 displacement leads to episulfone formation, with the less stable product preferentially formed. Here again reasonable alternative rationalizations have been proposed. l 5 , l 7 Acknowledgment. We thank L. Salem, M. Schlosser, and L. A. Paquette for some helpful discussions, A. Rossi for assistance with the calculations, and the National Institutes of Health (GM 13468) and the National Science Foundation (GP 28 137) for their support of our research. (14) Several such reactions have been discussed by M. Schlosser, who stresses a fundamental contrathermodynamic trend in carboncarbon linking reactions: BuU. SOC.Chim. Fr., 455 (1971); Top. Sfereochem., 5, 1 (1970). (15) N. P. Neureiter and F. G. Bordwell, J . Amer. Chem. Soc., 85, 1209 (1963); N. P. Neureiter, ibid., 88, 558 (1966). Reviewed by F. G. Bordwell in “Organosulfur Chemistry,” M. J. Janssen, Ed., Interscience, New York, N. Y., 1967, p 271, and by L. A. Paquette in “Mechanisms of Molecular Migrations,” Vol. 1, B. S. Thyagarajan, Ed., Interscience, New York, N. Y., 1968. p 121. (16) L. L. McCoy, J . Amer. Chem. Soc., 80, 6568 (1960); 82, 6416 (1962); 84, 2246 (1964); M. Mousseron, R. Fraise, R. Jacquier, and G. Bonavent, C. R . Acad. Sci., 248, 1465 (1959). (17) L. A. Paquette, Accounts Chem. Res., 1,211 (1968); D. J. Cram, “Fundamentals of Carbanion Chemistry,” Academic Press, NeH York, N. Y.,1965, p 253 (18) Fellow of the Alfred P. Sloan Foundation. Roald Hoffmann* Department of Chemistry, Cornell Unicersity Ithaca, New York I4850 Charles C. Levin Department of Chemistry, Smith College Northampton, Massachusetts 01060
Robert A. Mossla Wright Laboratory, School of Chemistry Rutgers University, The State Unicersity of New Jerse-v New Brunswick, New Jersey 08903 Receiced October I O , I972
A Direct Proton Magnetic Resonance Study of Hydrogen Bond Formation of Organic Lewis Acids with Triethylamine
Sir: The concept of the hydrogen bond has been widely applied to many chemical and biological systems, and consequently it has been the object of extensive discussion and review.’ Several workers recently determined thermodynamic quantities for hydrogen bond formation in numerous Lewis acid-base mixtures by calorimetric, 3-7 infrared, 2, 3,5,6,8-10 and fluorine- 19 nmr9- l 2 techniques. The enthalpies and equilibrium (1) G. C. Pimentel and A. L. McClellan, “The Hydrogen Bond,” W. H. Freeman, San Francisco, Calif., 1960. (2) E. M. Arnett, T. S. S. R. Murty, P. v. R. Schleyer, and L. Joris, J . Amer. Chem. Soc., 89,5955 (1967). (3) E. M. Arnett, L. Joris, E. Mitchell, T. S. A. R. Murty, T. M. Gorrie, and P. v. R. Schleyer, ibid., 92,2365 (1970). (4) E. M. Arnett and E. J. Mitchell, ibid., 93,4052 (1971). (5) K. F. Purcell, J. A. Stikeleather, and S. D. Brunk, ibid., 91, 4019 (1969). (6) R. S. Drago, N. O’Bryan, and G. C. Vogel, ibid., 92, 3924 (1970). (7) R. S. Drago, G. C. Vogel, and T. E. Needham, ibid., 93, 6014 (1971). (8) D. F. DeTar and R. W. Novak, ibid., 92,1361 (1970). (9) J. Mitsky, L. Joris, andR. W. Taft, ibid.,94,3442(1972). (10) R. W. Taft, D. Gurka, L. Joris, P. v. R. Schleyer, and J. W. Rakshys, ibid., 91,4801 (1969).
HFIP : TEA : Et,O
2.06 : 1.00: 30 t = -125OC
HFlP :TEA
il
I
J
1
ui HFIP: Et,O
“0
Figure 1. The proton magnetic resonance spectrum of the hydroxyl group of 1,1,1,3,3,3-hexafluoroisopropyl alcohol (HFIP) in a mixture of this compound with triethylamine (TEA) in excess diethyl ether (EtnO). The spectrum was recorded on a Varian HA-100 nmr spectrometer. The species responsible for the signals and the mole ratios of all components are shown in the diagram.
constants for hydrogen bond formation which they measured now provide a more quantitative basis for discussion of this process. For other acid-base systems, it has been demonstrated in these laboratories that at temperatures in the range of - loo”, ligand and proton exchange can be slowed sufficiently to permit the observation of separate pmr signals for bulk ligand and ligand molecules bound to diamagnetic cations’ 3- l5 and the boron trihalides.16-ls We have extended this method to acid-base systems in dilute solution and now wish to report the direct observation of a pmr signal for hydrogen nuclei involved in a hydrogen bond. This contrasts with the usual chemical shift studies of exchange averaged pmr signals. The alcohol portion of the room temperature pmr spectrum of a 1,1,1,3,3,3-hexafluoroisopropylalcohol (HF1P)-triethylamine (TEA) mixture in a large excess of ether consists of a hydroxyl group singlet positioned -7 ppm downfield from TMS and a multiple pattern for the 2-carbon hydrogen, split by the six fluorine neighbors. As the sample is cooled, the hydroxyl signal shifts far downfield, broadens extensively, even disappearing into the base line, and below - 1 lo”, it separates into two distinct peaks. The hydroxyl group signals for one of these mixtures, recorded at - 125 ” on a Varian HA-100 spectrometer, are shown in Figure 1. Analogous spectra were obtained with all solutions listed in Table I. Since the presence of water is precluded by the drying and purification procedures, l 9 the two signals in Figure 1 can be attributed to the hy(11) D. Gurka, R . W. Taft, L. Joris, and P. v. R. Schleyer, ibid., 89, 5957 (1967). (12) D. Gurka and R. W. Taft, ibid., 91,4794 (1969). (13) A. Fratiello, R. E. Lee, V. M. Nishida, and R. E. Schuster, J . Chem.Phys., 47,4951 (1967). (14) A . Fratiello, R. E. Lee, V. M. Nishida, and R. E. Schuster, ibid.. 48,3705 (1968). (15) A. Fratiello, R. E. Lee, V. M. Nishida, and R. E. Schuster, Inorg. Chem., 10,2552(1971). (16) A. Fratiello, T. P. Onak, and R. E. Schuster, J . Amer. Chem. Soc.,90,1194(1968). (17) A. Fratiello and R. E. Schuster, Inorg. Chem., 11, 1 1 (1972). (18) A. Fratiello and R. E. Schuster, J . Org. Chem., 37,2237 (1972). (19) PFP was sublimed, the other reagents were dried over CaH2 or CaSO4, and all samples were prepared in uacuo.
Communications to the Editor
632 Table I. Proton Magnetic Resonance Chemical Shift, Integration, and Kinetic Data for Lewis Acid-Base Hydrogen-Bond Complexes --Mole ratios-Acid :base :solventa
OH, 6, P P d
T,"C
Coordinated
Bulb
Complex mole ratio Acid :base
AH+, kcal/mol
-135
12.85
10.80
0.9:l.O
14
- 120 - 125 - 125
12.96 12.55 12.65
9.35 9.14 9.15
1.O:l.O 1.1 :1.0 1.O:l.O
18
-130 - 125
12.20 12.11
9.17 9.30
1.1:l.O 1.O:l. o
c -
HF1P:TEA:MeZO
2.10:1.00:37 HF1P:TEA:EtzO
3.10:1 .OO:13 2.71:1.00:29 2.06:1.00:38 P F P :TEA :Et20
1.95:1.00:34 2.02:1.00:39
a H F I P = 1,1,1,3,3,3-hexafluoroisopropyl alcohol; TEA = triethylamine; Me20 = dimethyl ether; E t 2 0 = diethyl ether; P F P = pfluorophenol. * Chemical shifts were measured with respect to internal TMS. The hydroxyl group signals arising from the TEA and ether adducts of the acids are designated as "coordinated" and "bulk" signals, respectively.
droxyl group of HFIP molecules in different electronic environments. The pmr concentration and area results of Table I indicate that the low-field signal in Figure 1 (and in analogous spectra for the other acid-base combinations listed) is due to the alcohol-TEA adduct and that at higher field arises from the remaining alcohol molecules hydrogen bonded to the ether. As seen in Table I signal areas showed that 1: 1 complexes were formed in each system. At 1: 1 mole ratios of Lewis acid to TEA, only the low-field signal was observed and at higher mole ratios the additional "bulk" proton signal became apparent. The calorimetric and spectroscopic studies of these systems indicate that at $25" complex formation is not complete,2~3~5~1',12 but using their thermodynamic data, the equilibrium constants at - 125" would be increased by several orders of magnitude. Thus, complete complexing of the base would be expected in these solutions. Using only pmr methods, it has not been possible to determine whether the TEA-acid adduct in these systems is a hydrogen-bonded complex, B. . .HA, or an ion pair, BH+. . .A-, resulting from complete proton transfer. The absence of N-H spin coupling could result from quadrupole relaxation, and it is not sufficient to rule out a proton-transfer process. For comparison to solutions of Table I, attempts were made to generate an R3NH+ cation in solution by dissolution of trialkylammonium salts or by the protonation of TEA with trichloroacetic acid. The fact that these attempts were unsuccessful because of solubility limitations may itself be evidence that the dominant process observed in the soluble systems of this study is hydrogen bonding. Recent ir and calorimetric data on TEA interactions with HFIP and PFP, in solvents of low dielectric constant, have been interpreted in terms of hydrogen bonding. l 2 Extensions of these ir measurements to mixtures of TEA with the much stronger acetic and benzoic acids8have led to a similar conclusion in contrast to earlier studies of these systems for which an ion-pairing mechanism was postulated. * O Since none of these studies were carried out in ether, several ir experiments were made in these laboratories on HFIP-TEA mixtures in this solvent, at concentrations comparable to those shown in Table I. The infrared spectra were recorded on a Beckman ?138z57310-
(20) G. M. Barrow and E. A . Yerger, J . Amer. Chem. SOC.,76, 5211 (1954).
Journal of the American Chemical Society j 95:2
IR-12 double-beam grating spectrophotometer using 0.015-mm, sealed KBr liquid cells. This short absorption path was necessary to avoid having the region in which the triethylammonium cation NH+ stretch absorbs (2700 cm-'), obscured by solvent absorption. Under these conditions hydrogen-bonded OH and OD stretching bands were easily identifiable as broad, moderately intense features at 3260 and 2420 cm-', respectively. No features attributable to the NH+ stretching motion of the triethylammonium cation or the COstretching motion of the hexafluoroisopropoxide anion were observed in these solutions. Thus, the infrared evidence indicates that hydrogen bonding is the dominant process in these HFIP-TEA mixtures at 25 '. Finally, the 19Fnmr method developed by Taft9-12 for investigating acid-base interactions was applied to a study of the p-fluorophenol (PFP)-TEA combination in ether.*' It has been demonstrated that the I9Fnucleus in PFP and other acids is an extremely sensitive probe to interactions at the hydroxyl functional group. For example, the formation of the p-fluorophenoxide ion in solution by complete proton transfer causes a 19Fnmr chemical shift displacement of 14 ppm from the position observed with PFP." The much smaller 19F chemical shifts (-2 ppm) exhibited by PFP in the presence of a variety of bases indicate that the hydroxyl proton is involved in weaker, hydrogen-bond intera c t i o n ~ . ~ -As ~ ~applied to one of the combinations of this study, solutions containing TEA, PFP, and ethyl ether in 0.5:1:30 and 1:1:30 mole ratios gave a lgF chemical shift displacement of 0.8 and 1.3 ppm, respectively, at - 1IO", relative to 1 :30 mole ratio mixture of PFP to ether.*' These small shifts result from a hydrogen-bonding interaction with proton transfer possibly occurring only to a small extent, perhaps -10Z. Since the concentration and temperature of the IgF nmr measurements closely approximate those reported in Table I, the conclusion can be drawn that the proton nmr data also reflect a hydrogen-bonding process. Although hampered by low signal intensities, the unknown contribution of viscosity broadening, and a small temperature range over which measurements could be made, approximate exchange data were obtained for HFIP-TEA in dimethyl ether and diethyl (21) The 19F nmr measurements reported here were carried out by Professor R. W. Taft at the University of California. Irvine. The authors are grateful to Dr. Taft for these experimental results and for helpful discussions concerning this paper.
January 24, 1973
633
ether by the usual line width technique. The approximate activation enthalpies shown in Table I are similar to the 15-20 kcal/mol generally observed for ligand exchange from cation solvation and larger than the 6-10 kcal/mol measured for proton exchange, for instance, in aqueous methylammonium ion, 25 AlC13, and imidazolium ion2' solutions. Similar pmr spectral measurements have been possible for other systems, including acetic acid, diethylamine, and pyridines, and attempts are being made to expand this list still further. Acknowledgments. This research was supported in part by a National Science Foundation Research Grant, GP-9517, a National Science Foundation instrument grant, GP-8347, for the purchase of the Varian HA-100 nmr spectrometer, a National Institutes of Health Research Career Development Award, 1K-4-GM-42334 (A. F.), and a Holy Cross College Faculty Fellowship (G. A. V).
Q0I3
24v26
(22) 2. Luz and S . Meiboom, J . Chem. Phys., 40,2686 (1964). (23) A. Fratiello, D. P. Miller, and R. E. Schuster, Mol. Phys., 12,111 (1967). (24) S. Thomas and W. L. Reynolds, Inorg. Chem., 9, 78 (1970). (25) E.Grunwald, A. Loewenstein, and S . Meiboom, J . Chem. Phys., 27,630 (1957). (26) D. W. Fong and E. Grunwald, J . Amer. Chem. SOC.,91, 2413 (1969). (27) E.I
